1. Field of the Invention
The field of the invention is structures that correct for thermal distortion in a device and more particularly, is MEMS (Micro Electro Mechanical Systems) and MOEMS (Micro Optical Electrical Mechanical Systems) structures that correct for thermal distortion in a device.
2. Background
The general class of microstructures referred to as Micro-Electro-Mechanical-Systems (MEMS) or Micro-Optical-Electrical-Mechanical-Systems (MOEMS) (hereinafter, MEMS and MOEMS are collectively referred to as xe2x80x9cMEMSxe2x80x9d) describes microstructures that are combined with micro-optical components for use in optical applications.
In recent years, driven by popularity of the Internet, the telecommunication industry has demanded products that will help increase communication bandwidth. As the speed and number of users increase, the industry recognizes that in order to increase the throughput of the system, it is desirable to transport more data by optical means, such as through optical fibers. Although a large amount of data is already communicated through fiber optic networks, a bottleneck that occurs is at the network junctions where data is switched between fibers. Since most switches currently in use are electrical, the optical data has to be converted from an optical signal to an electrical signal before switching and after switching, back to an optical signal whenever the signal crosses over a junction or switch. A need exists, therefore, to develop optical switches that can switch light without optical-electrical-optical conversion. A number of prior art methods are capable of redirecting light without such conversion. For example, please see the above cross reference to related patent applications invented by Ying Hsu and Arthur Telkamp. These methods include use of mirrors, light guiding structures, waveguides, liquid crystal and opto-mechanical elements.
One of the main challenges in the design of light guiding structure such as waveguides in optical switching is that temperature affects waveguide alignment. A poor alignment may result in an excessive loss of light such that the data is attenuated below a useful level. In conventional applications, waveguides are deposited on silicon substrates. These substrates are thick (e.g., 400 to 500 microns) and can withstand the stress due to mismatch of the CTE (Coefficient of Thermal Expansion) of the waveguide material (typically, for example, silica or polymer) and the CTE of the silicon substrate. However, in applications where the silicon substrate is thinned to accommodate other purposes, the resulting stress due to temperature can cause a physical distortion such that the waveguides above the silicon will not align properly. The CTE for silicon is 2.3 parts per million parts per degree of Celsius, while for silica, the CTE is 0.5. Distortion of the substrate results when two materials are combined at a temperature that is different than the temperature for which the combination is used. For example, silica is typically deposited at 350xc2x0 C., but the finished device is used at 23xc2x0 C. or room temperature. The 327xc2x0 C. difference is sufficient to exert enough thermal stress to distort a thin silicon structure. For some material, the deposition temperature is not the highest temperature of interest. Oxynitride, for example, is a material used to fabricate waveguides and requires a high temperature anneal at 1100xc2x0 C. As a result, using Oxynitride on a thin substrate can result in substantial thermal distortion.
The thermally induced distortion of a physical structure due to a CTE mismatch is an old problem that the prior art has attempted to solve in a number of ways. In certain cases, thermal distortion is actually desirable. For example, a well known example of desirable thermal distortion is a xe2x80x9cbi-metalxe2x80x9d structure whereby two strips of dissimilar metal are bonded into a single assembly. When the finished unit is subject to a temperature change, the resulting thermal stress induced by the difference in expansion will bend the strips; the degree of bending is used as an indication of temperature. These devices are often still used today as temperature sensors in many home thermostats.
In the design of high performance mirrors for optical networks, however, the thermal distortion of a bi-metal is a serious problem that has to be overcome. These mirrors require a metal coating on top of the substrate to increase reflectivity. The result is that the finished mirror will distort when subjected to a temperature change. The solution to eliminating the distortion is accomplished generally by coating the opposite side of the substrate with the same material as the reflective coating such that an opposing stress will cancel the stress from the reflective coating, thus resulting in negligible effective distortion. This approach is referred to as a balanced stress method.
The balanced stress method has limited application to MEMS structures. The few, but widely used, surface and bulk micro-machining processes do not easily permit one to place an equal layer on the opposite side of a structure. MEMS design actually seeks to minimize the number of layers of material in order to reduce fabrication cost and increase yield. In the case of waveguides, the placement of waveguides on the opposite side of a substrate will greatly increase the complexity of the manufacturing process. Where waveguides are placed on top of thin and suspended silicon structures, the CTE mismatch can cause severe thermal distortions.
Therefore, there is a need for a microstructure that is able to maintain a precise optical alignment over a large temperature range.
The invention relates generally to novel microstructures and methods that compensate for or correct thermal distortion in an optical device.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.